The worldwide acceptance of the economic benefits and high weld quality produced when using solid state joining techniques, including for example conventional rotary friction, to produce joints in round section metallic components has led to the development of other high weld quality processes for joining non-round and complex shaped parts.
One newly developed process related to rotary friction welding, called linear friction welding (“LFW”), involves rubbing one component across the face of a second rigidly clamped component using a balanced, linear reciprocating motion. The linear reciprocating motion generates frictional heat and softening of the material at the weld interface that is expelled as flash. The two components are brought into perfect alignment towards the end of the weld cycle, and the welding force is maintained or increased to consolidate the joint. Machining or grinding can subsequently remove flash produced during the LFW process. The advantage to the LFW process over conventional rotary friction processes is that non-round or complex geometry components, such as aircraft engine blades to discs, can be welded using LFW.
Friction stir welding (“FSW”) is a solid-state process that uses a nonconsumable tool to join various types of metals. When a FSW rotating tool is inserted into and traverses through the materials, the tool plasticizes the materials and forces the materials to flow around the tool where they reconsolidate.
While processes such as FSW and LFW have found popularity in various industries, the processes do have drawbacks. Age-, work-, or strain-hardened metals exposed to the intense localized heat of welding tend to recrystallize and soften in the weld nugget because the strengthening precipitates have dissolved. The weld nugget can be strengthened by performing a post weld aging treatment that reprecipitates these strengthening precipitates. Welding also results in a heat-affected zone (HAZ) that surrounds the nugget. Whereas the nugget attains a high enough temperature to cause the precipitates to solutionize, the HAZ is heated to a lower temperature that causes the precipitate to grow in size and become less effective in strengthening. Post weld aging only further coarsens these precipitates.
For example, both FSW and LFW reduce the strength of commonly used aluminum alloys (such as 7050 and 7055) by locally altering the microstructure and temper (In friction welded aluminum, for example, the friction welding process reduces the microstructure to a grain size of about 2-5 micrometers, which is much finer than the parent material). In fact, a strength degradation of about 25-30% typically occurs because of overaging in the HAZ. The nugget will initially be weaker, but will gradually strengthen by virtue of natural aging at room temperature.
For example, as shown in Table 1 below, for one type of aluminum alloy (7050-T7451), the linear friction welding and subsequent aging resulted in a drop in allowable ultimate tensile stress (Ftu) of about 15%, a drop in the allowable tensile stress at which the material starts to yield in tension (Fty) of about 27%, and a drop in percent elongation of about 35-45%.
ConditionFty (ksi)Ftu (ksi)% Elongation7050-T745163747.5Parent Metal7050-T745146634–5Linear FrictionWelded and Aged
It is impossible or impractical to restore these strength properties by further mechanical processing, which severely limits the usefulness and applicability of these aluminum alloys in low-cost manufacturing process, especially for use in airframe components.
One technique for recovering the strength of these aluminum alloys altered during the FSW or LFW process is to further heat treat the alloys using solution heat treating (at temperatures between about 900 and 1000 degrees Fahrenheit), quenching and aging (at temperatures ranging from about 250-350 degrees Fahrenheit). However, while the solution heat treatment step is necessary for the recovery of strength, it also results in excessive growth of the grains (from about 4 to 400 micrometers) of the weld itself and significantly degrades ductility, frequently to levels approaching zero.
This excessive grain growth and loss of ductility is thought to occur because the friction welded blocks contain considerable amounts of stored energy, which causes secondary recrystallization and abnormal grain growth to occur during the solution treatment step.
It is thus highly desirable to recover the strength of aluminum alloys lost during FSW, LFW or similar solid-state processes while preventing the resultant loss in ductility.